Lamellar-structure titanium-aluminum based alloy having a beta-gamma phase

ABSTRACT

A lamellar titanium-aluminium (TiAl) alloy having a beta-gamma phase according to the present invention contains aluminum (Al) of 40˜46 at %, niobium (Nb) of 3˜6 at %, a creep resistance enhancer of 0.2˜0.4 at %, a softening resistance enhancer of 2 at %, and the balance of titanium (Ti) and is manufactured by vacuum arc melting.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a lamellar titanium-aluminum (TiAl) based alloy having a beta-gamma phase, and more particularly, to a lamellar titanium-aluminum (TiAl) alloy having a beta-gamma phase of which the effect of stabilizing a beta phase is improved and the manufacturing cost is reduced, by adding tungsten (W) and chromium (Cr) that are not expensive instead of molybdenum (Mo) and vanadium (V) that are usually added to stabilize a beta phase.

The present invention relates to a lamellar titanium-aluminum (TiAl) alloy having a beta-gamma phase of which mechanical properties are improved by adding a small amount of carbon (C) and silicon (Si) that are effective in grain refinement and creep resistance to suppress production of a deposit and of which high-temperature oxidation resistance and ductility are improved by adding niobium (Nb).

2. Description of the Related Art

A titanium-aluminum (TiAl) based alloy, which is the next generation light heat-resistant material as a kind of intermetallic compounds, is a two-phase alloy containing Ti₃Al of 10%.

A lamellar structure ingot having two phases of TiAl(γ)+Ti3Al(α₂) is obtained by common melting-solidifying.

The titanium-aluminum (TiAl) lamellar structure has been known as providing properties that are useful for the practical use of titanium-aluminum (TiAl) as a light high-temperature material, because it has high fracture toughness, fatigue strength, and creep strength, but its insufficient ductility at a room temperature has been known as the largest obstacle in use as a casting material.

It has been known that the most important factor for the insufficient ductility is delamination on the interface when stress is exerted perpendicular to the lamellar boundary.

The rough grains also reduce the ductility. Accordingly, high strength and ductility, in addition to excellent high-temperature properties, can be obtained if it is possible to reduce the grain size and contain beta and gamma phases having high ductility relative to a lamellar structure.

The existing studies have reported that they have used Ti-(41˜45)Al-(3˜5)Nb—(Mo,V)—(B,C)-based alloys to manufacture a lamellar titanium-aluminum (TiAl) alloy having beta and gamma phases (H. Z. Niu et al, Intermetallics 21 (2012) 97 and T. Sawatzky, Y. W. Kim et al., Mat. Sci. Forum 654-656 (2010) 500).

SUMMARY OF THE INVENTION

An object of the present invention is to provide a lamellar titanium-aluminum (TiAl) alloy having a beta-gamma phase of which the effect of stabilizing a beta phase is improved and the manufacturing cost is reduced, by adding tungsten (W) and chromium (Cr) that are not expensive instead of molybdenum (Mo) and vanadium (V) that are usually added to stabilize a beta phase.

Another object of the present invention is to provide a lamellar titanium-aluminum (TiAl) alloy having a beta-gamma phase of which mechanical properties are improved by adding a small amount of carbon (C) and silicon (Si) that are effective in grain refinement and creep resistance to suppress production of a deposit and of which high-temperature oxidation resistance and ductility are improved by adding niobium (Nb).

An aspect of the present invention provides a lamellar titanium-aluminum (TiAl) alloy having a beta-gamma phase which contains aluminum (Al) of 40˜46 at %, niobium (Nb) of 3˜6 at %, a creep resistance enhancer of 0.2˜0.4 at %, a softening resistance enhancer of 1˜3 at %, and the balance of titanium (Ti) and is manufactured by solid-casting.

The creep resistance enhancer may include one or more of carbon (C) and silicon (Si).

The softening resistance enhancer may be any one selected from tungsten (W) and chromium (Cr).

The average hardness may be 335.6 Hv or more.

Young's modulus may be 180˜220 GPa.

Tensile strength may be 453.8˜540 MPa.

According to the present invention, stabilization of a beta phase is maximized by adding tungsten (W) and chromium (Cr) that are inexpensive elements instead of molybdenum (Mo) and vanadium (V) that are usually added to stabilize a beta phase.

Further, mechanical properties are improved by adding a small amount of carbon (C) and silicon (Si) that are effective in grain refinement and creep resistance to suppress production of a deposit.

Further, high-temperature oxidation resistance and ductility are improved by adding niobium (Nb) and the manufacturing cost is reduced.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a picture of the external appearances of lamellar titanium-aluminum (TiAl) alloys having a beta-gamma phase according to the present invention.

FIG. 2 is a table showing the components of lamellar titanium-aluminum (TiAl) alloys having a beta-gamma phase according to the present invention and a comparative example.

FIG. 3 is a picture of a fine structure in a lamellar titanium-aluminum (TiAl) alloy having a beta-gamma phase according to a first embodiment of the present invention.

FIG. 4 is a picture of a fine structure in a lamellar titanium-aluminum (TiAl) alloy having a beta-gamma phase according to a second embodiment of the present invention.

FIG. 5 is a picture of a fine structure in a lamellar titanium-aluminum (TiAl) alloy having a beta-gamma phase according to a third embodiment of the present invention.

FIG. 6 is a picture of a fine structure according to a comparative example.

FIG. 7 is a table comparing tensile strengths of the lamellar titanium-aluminum (TiAl) alloys having a beta-gamma phase according to the second and third embodiments of the present invention and a comparative example.

FIG. 8 is a table comparing Vickers hardness of the lamellar titanium-aluminum (TiAl) alloys having a beta-gamma phase according to the first to third embodiments of the present invention and a comparative example.

FIG. 9 is a graph comparing the result of tensile test on the lamellar titanium-aluminum (TiAl) alloys having a beta-gamma phase according to the second and third embodiments of the present invention and a comparative example.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

A lamellar titanium-aluminum (TiAl) alloy having a beta-gamma phase according to the present invention is described hereafter with reference to FIGS. 1 and 2.

The terms and words used in the present specification and claims should not be interpreted as being limited to typical meanings or dictionary definitions, but should be interpreted as having meanings and concepts relevant to the technical scope of the present invention based on the rule according to which an inventor can appropriately define the concept of the term to describe most appropriately the best method he or she knows for carrying out the invention.

Therefore, the configurations described in the embodiments and drawings of the present invention are merely most preferable embodiments but do not represent all of the technical spirit of the present invention. Thus, the present invention should be construed as including all the changes, equivalents, and substitutions included in the spirit and scope of the present invention at the time of filing this application.

FIG. 1 is a picture of the external appearances of lamellar titanium-aluminum (TiAl) alloys having a beta-gamma phase according to the present invention and FIG. 2 is a table showing the components of lamellar titanium-aluminum (TiAl) alloys having a beta-gamma phase according to the present invention and a comparative example.

The lamellar titanium-aluminum (TiAl) alloy having a beta-gamma phase (hereafter, referred to as a titanium-aluminum (TiAl) alloy 10) according to the present invention shown in the figure has been manufactured by performing solid-casting on the metallic components shown in FIG. 2 and have not undergone post processes such as heat treatment, hot isostatic pressing, rolling, and forging.

In detail, it is apparent that the hardness, softening resistance, and creep resistance of the titanium-aluminum (TiAl) alloy 10 are improved, when post processes such as heat treatment are applied, but in the present invention, hardness and tensile tests were performed on an embodiment having the shape of a button manufactured by solid-casting and having a diameter of 60 mm and was compared with a comparative example.

The comparative example is based on the components of the titanium-aluminum (TiAl) heat resisting alloy disclosed in JP-A-10-220236 and JP-A-10-193087 by Daido Steel, Japan.

The present invention may be classified into first to third embodiments.

The first embodiment was manufactured by changing the amounts of aluminum (Al) and niobium (Nb) with the composition ratio of tungsten (W) and carbon (C) maintained at a predetermined level, the second embodiment was manufactured by changing the amount of niobium (Nb), and the third embodiment was manufactured by changing the amounts of aluminum (Al) and niobium (Nb) with the amounts of chromium (Cr), silicon (Si), and carbon (C) maintained at a predetermined level.

The composition of the TiAl alloy 10, based on Table 2, contains aluminum (Al) of 40˜46 at %, niobium (Nb) of 3˜6 at %, a creep resistance enhancer of 0.2˜0.4 at %, a softening resistance enhancer of 1˜3 at %, and the balance of titanium.

The creep resistance enhancer contains one or more of carbon (C) and silicon (Si), any one of tungsten (W) and chromium (Cr) is selected as the softening resistance enhancer, and the alloy has an average hardness over 335.6 Hv and a Young's modulus of 180˜220 GPa, when post processes such as heat treatment is not performed yet.

The fine structures of the first to third embodiments of the present invention and a comparative example are described hereafter with reference to FIGS. 3 to 6.

FIGS. 3 to 5 are pictures of fine structures in a lamellar titanium-aluminum (TiAl) alloy having a beta-gamma phase according to first to third embodiments of the present invention and FIG. 6 is a picture of a fine structure of a comparative example.

The fine structures shown in FIG. 3 were manufactured when tungsten (W) was selected as a softening resistance enhancer and carbon (C) was added as a creep resistance enhancer, and the all have a lamellar structure.

The fine structures shown in FIG. 4 were manufactured when tungsten (W) was selected as a softening resistance enhancer and silicon (Si) was added as a creep resistance enhancer, and they also have a lamellar structure.

The fine structures shown in FIG. 5 were manufactured when chromium (Cr) was selected as a softening resistance enhancer and carbon (C) and silicon (Si) were added as creep resistance enhancers, and they have a lamellar structure.

The titanium-aluminum (TiAl) alloy according to a comparative example, as shown in FIG. 6, has a low grain size, but does not show a clear lamellar structure in the grains and a vulnerable α₂ (Ti₃Al) phase is distributed along the grain boundary.

The embodiments of the present invention and the comparative example have a difference in strength, as shown in FIG. 7, due to the differences in fine structure described above.

That is, FIG. 7 is a table comparing tensile strengths of lamellar titanium-aluminum (TiAl) alloys having a beta-gamma phase according to the second and third embodiments of the present invention and a comparative example. It could be seen that the embodiments of the present invention had a tensile strength over 453.8 MPa, while the comparative example had a tensile strength of 384.5 MPa, so the strength of the titanium-aluminum (TiAl) alloys according to the present invention was far superior.

FIG. 8 is a table comparing Vickers hardness of lamellar titanium-aluminum (TiAl) alloys having a beta-gamma phase according to the first to third embodiments of the present invention and a comparative example.

As shown in FIG. 8, the specimens of first to third embodiments of the present invention showed an average hardness over 335.6 Hv, as the result of measuring Vickers hardness three times for each of the specimens.

In contrast, it was seen that the specimen of the comparative example had hardness of 268.4 Hv, considerably lower than those of the embodiments of the present invention.

Finally, FIG. 9 is a graph comparing the result of tensile test on the lamellar titanium-aluminum (TiAl) alloys having a beta-gamma phase according to the second and third embodiments of the present invention and a comparative example. As the result of performing a tensile test on #7 of the second embodiment, #11 of the third embodiment, and the comparative example, they showed Young's modulus of 180˜220 GPa.

The test results correspond to the first to third embodiments manufactured by performing solid-casting on the titanium-aluminum (TiAl) alloy having the composition shown in FIG. 2, without post processes such as heat treatment and plastic working performed.

Accordingly, Young's modulus, hardness, and tensile strength can be further improved if post processes are additionally performed. Further, if the added amount is changed with the composition range shown in FIG. 2 and a softening resistance enhancer and a creep resistance enhancer are selectively added, it is apparent that softening resistance and creep resistance are selectively increased and desired properties can be obtained.

The scope of the present invention is not limited to the embodiments described above and many other modifications based on the present invention may be achieved by those skilled in the art within the scope of the present invention.

In the present invention, molybdenum (Mo) and vanadium (V) that are usually added to stabilize a beta phase were replaced by tungsten (W) and chromium (Cr) that are inexpensive elements.

Accordingly, it is possible to reduce the manufacturing cost and maximize the effect of stabilizing a beta phase.

Further, it is possible to manufacture a lamellar titanium-aluminum (TiAl) alloy having a beta-gamma phase of which mechanical properties are improved by adding a small amount of carbon (C) and silicon (Si) for grain refinement to suppress production of a deposit and of which high-temperature oxidation resistance and strength are improved by adding niobium (Nb). 

1. A lamellar titanium-aluminum (TiAl) alloy having a beta-gamma phase that contains aluminum (Al) of 40˜46 at %, niobium (Nb) of 3˜6 at %, a creep resistance enhancer of 0.2˜0.4 at %, a softening resistance enhancer of 1˜3 at %, and the balance of titanium (Ti) and is manufactured by solid-casting to improve softening resistance and creep resistance.
 2. The lamellar titanium-aluminum (TiAl) alloy of claim 1, wherein the creep resistance enhancer includes one or more of carbon (C) and silicon (Si).
 3. The lamellar titanium-aluminum (TiAl) alloy of claim 2, wherein the softening resistance enhancer is any one selected from tungsten (W) and chromium (Cr).
 4. The lamellar titanium-aluminum (TiAl) alloy of claim 3, wherein average hardness is 335.6 Hv or more.
 5. The lamellar titanium-aluminum (TiAl) alloy of claim 1, wherein Young's modulus is 180˜220 GPa.
 6. The lamellar titanium-aluminum (TiAl) alloy of claim 5, wherein tensile strength is 453.8˜540 MPa.
 7. The lamellar titanium-aluminum (TiAl) alloy of claim 2, wherein Young's modulus is 180˜220 GPa.
 8. The lamellar titanium-aluminum (TiAl) alloy of claim 3, wherein Young's modulus is 180˜220 GPa.
 9. The lamellar titanium-aluminum (TiAl) alloy of claim 4, wherein Young's modulus is 180˜220 GPa. 